ck_brlock: Serialize on atomic operation on read acquisition.

10% improvement for reader acquisition latency on TSO architectures
implementing wait-free FAS.

include/ck_brlock.h

@@ -187,10 +187,10 @@ ck_brlock_read_lock(struct ck_brlock *br, struct ck_brlock_reader *reader)
 		while (ck_pr_load_uint(&br->writer) == true)
 			ck_pr_stall();
 
-		ck_pr_store_uint(&reader->n_readers, 1);
+		ck_pr_fas_uint(&reader->n_readers, 1);
 
-		/* Loads are re-ordered with respect to prior stores. */
-		ck_pr_fence_strict_memory();
+		/* Serialize counter update with respect to writer snapshot. */
+		ck_pr_fence_memory();
 
 		if (ck_pr_load_uint(&br->writer) == false)
 			break;